Remote keyless entry (RKE) systems are extensively used in vehicles to provide a convenient way to lock and unlock vehicle doors. Tire pressure monitoring systems (TPMS) are also being incorporated into vehicles to provide a convenient way of tracking tire pressure and notifying a user if any of the tires is experiencing low pressure. The RKE and TPMS functions may be combined into a single circuit.
The circuit will draw current even when the vehicle is turned off because it needs to be able to receive a key fob signal at any time to assess whether to unlock the vehicle door. The circuit also should be able to receive a tire pressure monitoring signal so that a user will be able to detect a low tire pressure condition as soon as it occurs. Thus, it is desirable for the circuit to have a low biasing current draw to prevent the vehicle battery from draining. This may be difficult, however, because the circuit has both a controller and receiver that must be operating nearly continuously to detect the key fob signal, thereby constantly drawing current even when the vehicle is turned off.
It is possible to reduce the overall current draw by shutting down the controller in the circuit and waking up the controller to check for a key fob signal and/or a tire pressure signal at predetermined time periods dictated by a continuously running clock because the clock itself draws very little current. To do this, the circuit may be duty-cycled in the most efficient manner possible to minimize current consumption while still guaranteeing reliable reception of TPMS and RKE data.
Every time a receiver in the circuit detects data, the controller will wake up. However, because data and noise appear alike to the receiver, using a standard synchronous duty cycle that checks for TPMS data and RKE data at the same time caused an excessive number of false wakeups in the controller due to noise output by the receiver. This is because TPMS data may need a longer verification period than RKE data and, therefore, the controller needs to be kept awake for a longer period of time to detect the TPMS data, increasing the chance that the receiver will also pick up noise that could be mistaken as data. Even though the noise will be screened out by the controller after it wakes up, these false wakeups cause the controller to draw excessive current unnecessarily. But reducing the number of actual controller wakeups may reduce the reliability of data reception.
There is a desire for a TPMS/RKE system that reduces current draw without sacrificing data reception reliability.